The plant hormone auxin, typified by indole-3-acetic acid (IAA) regulates various aspects of plant growth and development such as cell division, vascular differentiation, lateral root formation, apical dominance, morphogenesis, oncogenesis, tropisms, and is generally considered to be responsible for regulating plant cell growth. Despite the importance of auxin in plant development, the primary molecular mechanism of auxin action is unknown. Experimental evidence accumulated in the last 10 years indicates that the hormone acts rapidly (3-5 min) at the transcriptional level. Molecular, biochemical, genetic, and structural studies using early auxin inducible genes from Pisum sativum (pea) and Arabidopsis are proposed herein to gain insight into: (i) the molecular details of the signaling apparatus responsible for their transcriptional activation; and (ii) the role of their encoded polypeptides in plant growth and development. The specific aims of the proposal are: 1. To isolate the transcriptional factor(s) that interact with the auxin responsive domain A (AuxRD A) of the early PS-MA 4/5 gene using molecular genetic ("one-hybrid system" in yeast) or biochemical (DNA affinity chromatography) approaches. To clone and structurally characterize genomic sequences encoding this factor and determine its expression characteristics during plant development. 2. To isolate mutations in Arabidopsis that affect gene expression driven by AuxRD A using Arabidopsis lines already constructed expressing GUS via these domains. To done one of these mutations by positional cloning. 3. To biochemically characterize the PS-IAA4/5-like proteins and define the DNA sequences that bind at the putative beta-alpha-alpha DNA binding domain of these nuclear short-lived proteins. To isolate genomic sequences that contain the binding site (target/late genes). The elucidation of the primary mechanism of auxin action is of paramount importance in Plant Sciences. Understanding how auxin controls cell growth, meristem activity, xylogenesis, etc. will allow the engineering of plant species with improved properties and qualities.